1. Field of the Invention
The present invention relates to a tool for forming a hole in macroporous compressible soil and to a method of forming a hole by the same tool.
The invention may be utilized while forming holes in the soil specified for cast-in-place piles in industrial and civil engineering practice.
For the purpose of the present invention by "macroporous compressible soil" is meant loess, water-saturated sandy soil, soft water-saturated clay soil with a consistency index I.sub.L .gtoreq.0.3 and other similar kinds of soil.
Such soil is known for its low structural strength, it easily collapses in contact with water and under dynamic load.
Conventional methods of forming holes in soil such as percussion and rotary drilling, washboring as well as tools for practicing such methods are practically of low efficiency for the soil specified. Thus, for example, the percussion drilling destroys the wall of a hole. Combined methods of percussion drilling and vibratory action, apart from the abovementioned disadvantage, has an effect on nearby buildings and produces noise. Since the percussion tools operate by a sequence of pulses their efficiency is limited. The method of rotary drilling does not provide for an adequate compaction of the wall of the hole. Therefore such walls have an inferior performance characteristics, which in turn results in a lower carrying capacity of the pile. The washboring of a hole in the soil specified is inappropriate in general.
Furthermore, conventional methods of forming holes singly or in combination to a greater or lesser degree involve soil excavation but this is not known to be any help in compacting the wall of the hole.
2. Description of the Prior Art
Known in the art are a tool and method of forming a hole in macroporous compressible soil (U.S. Pat. No. 4,193,461). The tool comprises a body adapted to be connected to a boring rod and having a sizing part and coaxial body portions radially defined by soil compaction surfaces and of step-down radii from the sizing part to the tool tip.
Each body portion is constructed as a ram assembly comprising one or more ram shoes radially movable from a retracted position to an extended position. As the tool is driven into soil each body portion circumferentially compacts the soil to the radial extent which is equal to or slightly greater than the radius of an adjacent body portion in the direction from the tool tip to the sizing part when the ram shoe or shoes are in the retracted position. In such a manner the hole is formed simultaneously with the soil compaction operation.
The prior art tool provides for soil compaction by a discontinuous radial action on the soil with the ram shoes in zones corresponding to the body portions. Therefore a nonuniform compaction of the soil both circumferentially of the hole and along its length is the result which has an effect on the carrying capacity of the pile. Such a discontinuous action on the soil, which offers an increased resistance to such action as it is compacted, is by the reciprocating ram shoes. Such a construction of the tool as well as the method of acting on the soil are limited as to efficiency and are characterized by an increased consumption of power, since the ram shoes are to be retracted prior to effecting the next power stroke.